Spending less time in long, uninterrupted sitting bouts and replacing some of that time with even light physical activity was associated with lower cancer risk in a study of more than 91,000 UK Biobank participants, highlighting how patterns of sedentary behavior may matter alongside total sitting time.
Study: Accelerometry-measured prolonged and interrupted sedentary behavior and cancer incidence and mortality: A cohort study of 91,292 UK Biobank participants. Image credit: ThalesAntonio/Shutterstock.com
A large study published in PLOS Medicine found that long, uninterrupted bouts of sitting were more strongly associated with cancer risk than sedentary time interrupted by even light physical activity.
Prolonged sitting may influence cancer risk
Sedentary behavior, defined as waking activities that involve an energy expenditure of 1.5 or fewer metabolic equivalents of task (METs) while sitting, reclining, or lying down, accounts for an estimated 55% of waking hours. A growing body of evidence suggests that spending more time sedentary is associated with poorer health outcomes, including a higher risk of several chronic diseases and cancer-related death.
Some studies have also found that replacing sedentary time with physical activity may be associated with lower cancer mortality, potentially through improvements in glucose metabolism and insulin regulation that occur independently of body weight.
However, it remains unclear whether the accumulation of sedentary time matters as much as the amount of time spent sitting. Current public health guidance focuses primarily on reducing total sedentary time because there is limited evidence on the most effective way to do so. In particular, it is unknown whether breaking up long periods of sitting with brief bouts of activity provides greater benefits than simply increasing overall physical activity. It has also been unclear whether these associations differ according to obesity status.
To address these questions, the researchers compared the cancer risks associated with prolonged, uninterrupted sedentary behavior and shorter, interrupted sedentary bouts, and examined whether replacing prolonged sitting with physical activity was associated with lower cancer risk.
Researchers tracked sitting patterns in 91,000 adults
The researchers analyzed data from 91,292 UK Biobank participants (56% women), aged 37 to 73 years, who wore wrist accelerometers continuously for seven days to objectively measure their activity patterns. Participants were then followed for a median of 12.4 years to examine associations between sedentary behavior and cancer outcomes.
To distinguish different sitting patterns, sedentary behavior was classified as prolonged when a bout lasted at least 30 minutes and at least 90% of that time was spent sedentary. All other sedentary bouts were classified as interrupted. Older participants generally accumulated more total sedentary time than younger participants.
Differences in lifestyle characteristics also emerged across sedentary behavior patterns. Participants with higher levels of both total and prolonged sedentary time were more likely to experience greater socioeconomic deprivation, have higher educational attainment, and consume red and processed meat more frequently. In contrast, those who accumulated more interrupted sedentary time tended to eat slightly more fruit and vegetables and slightly less red meat.
Over the follow-up period, participants in the highest quartile of total sedentary time had higher rates of cancer mortality, obesity-related cancers, and type 2 diabetes-related cancers than those in the lowest quartile. Higher rates were also observed for several individual cancers, including esophageal, kidney, liver, colorectal, lung, and prostate cancers.
How sedentary time accumulated changed cancer risk
Overall, spending more time sedentary was associated with a greater risk of cancer. Each additional hour of total sedentary time was associated with a 12% higher risk of cancer death, alongside a 6%–7% higher risk of obesity-related and type 2 diabetes-related cancers.
However, separating sedentary time into prolonged and interrupted patterns revealed an important difference. The increased risks were driven entirely by prolonged, uninterrupted sitting: each additional hour of prolonged sedentary behavior was associated with a 9% higher risk of cancer death and a 5% higher risk of obesity-related and type 2 diabetes-related cancers.
By contrast, sedentary time accumulated in shorter, interrupted bouts showed the opposite pattern. Each additional hour of interrupted sedentary behavior was associated with an approximately 18%–19% lower risk of cancer death and around a 10% lower risk of obesity-related and type 2 diabetes-related cancers. These apparent benefits were greatest among participants with lower overall levels of interrupted sedentary behavior, then leveled off at higher levels.
The researchers also observed similar patterns across several individual cancer types, with prolonged sedentary behavior generally associated with higher risks and interrupted sedentary behavior with lower risks.
Replacing prolonged sitting with activity lowered associated risk
When one hour of prolonged sedentary behavior was replaced with physical activity, the associated cancer death risk was 12% lower for light activity. With 30 minutes of moderate physical activity instead, the associated risk was 8% lower. The largest reduction in associated risk was observed when five minutes of vigorous physical activity replaced prolonged sedentary activity, although this association was statistically significant for cancer incidence outcomes rather than cancer mortality.
Similar substitution patterns were observed across several composite and site-specific cancer outcomes, although not all associations reached statistical significance.
Prolonged sitting as a prevention target
These findings suggest that not only the total amount of sedentary time, but also how sedentary time is accumulated, may be important for cancer risk.
Replacing prolonged sedentary time with even light physical activity was associated with significantly lower cancer risks in the prolonged sedentary behavior subgroup, suggesting a potential intervention target for future studies.
This is among the earliest studies to suggest that replacing prolonged sedentary behavior with physical activity is associated with lower cancer risks.
Possible mechanisms include improved metabolism and lower inflammation, as well as better immune function, with physical activity in interrupted sedentary behavior. In addition, the authors cite other studies that suggest harmful patterns of fat deposition in the heart and liver, for instance, with sedentary lifestyles, as well as impaired glucose and insulin regulation. Persistently high insulin levels can promote cell proliferation and have been linked to several cancers.
The researchers also found no evidence that these associations differed according to general or central obesity, suggesting the observed relationships were broadly similar across obesity groups.
One week of activity may not reflect habits
The study used objective measures of activity and sedentary behavior, while categorizing sedentary behavior into prolonged and interrupted types for the purpose of analyzing associations with overall and individual cancer risks.
However, the observational nature of the study precludes causal interpretation. The UK Biobank participants are volunteers and may not represent the general population. Healthy volunteer bias was found to be likely. Moreover, the study covered only a limited period of activity, which may not reflect long-term sedentary behavior. Additionally, it did not identify the type of sedentary behavior involved, such as viewing television or driving, or sitting at work.
How sitting time is accumulated may influence cancer risk
Overall, the findings suggest that cancer risks associated with sedentary behavior may depend not only on how much time people spend sitting, but also on how that sedentary time is accumulated. Long, uninterrupted bouts of sitting were consistently associated with higher cancer risks, while replacing some of this time with physical activity was associated with lower cancer-related risks.
Although the observed effect sizes were modest, the authors note that they could still have meaningful implications at the population level given how common sedentary behavior is. However, because this was an observational study, the findings cannot establish cause and effect, and intervention studies will be needed to confirm whether reducing prolonged sedentary behavior directly lowers cancer risk.
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